Method for making cellular rubbery polymer

ABSTRACT

1. METHOD FOR MAKING A GAS-FREE CELLULAR RUBBERY POLYMER SPONGE HAVING OPEN CELLS WHICH COMPRISES, COLD MIXING AND MILLING A CURABLE RUBBERY POLYMER, A CURING AGENT FOR CURING SAID POLYMER, AND 5 TO 30% BY WEIGHT OF AN ORGANIC LIQUID BLOWING AGENT BASED ON THE WEIGHT OF THE POLYMER, SAID ORGANIC LIQUID BLOWING AGENT BEING A LIQUID HAVING A BOILING POINT IN THE RANGE OF 70* TO 200* C. AND AT LEAST 5*C. BELOW THE CURING TEMPERATURE OF THE POLYMER, AND BEING SELECTED FROM THE GROUP CONSISTING OF AROMATIC HYDROCARBONS. NAPHTHENIC HYDROCARBONS. PARAFFINIC HYDROCARBONS, FLUORINE-SUBSTITUTED PARAFFINS, CHLORINE-SUBSTITUTED PARAFFINS, AND KETONES FREE OF DISSOLVED GAS AND COMPATIBLE WITH THE POLYMER TO COMPOUND THE RUBBERY POLYMER WITH THE CURING AGENT AND SAID ORGANIC LIQUID BLOWING AGENT DISSOLVED INOT THE POLYMER AS PLASTICIZER, CURING THE RESULTING COMPOUNDED POLYMER AT A CURING TEMPERATURE IN THE RANGE OF 120* TO 205*C. WITH SIMULTANEOUS BLOWING OF THE POLYMER UNDERGOING CURING BY VOLATILIZATION OF SAID ORGANIC LIQUID BLOWING AGENT UNDER ABOUT ATMOSPHERIC PRESSURE FOR ABOUT FIVE MINUTES TO AN HOUR, THE CURING AND BLOWING OF THE COMPOUNDED POLYMER BEING THUS CARRIED OUT IN A MOLD CAVITY IN WHICH THERE IS ROOM FOR AT LEAST 50% EXPANSION SO THAT THE VOLATILIZED ORGANIC LIQUID BLOWING AGENT FORMS OPEN CELLS IN THE RESULTING SPONGE MATRIX OF THE CURED POLYMER MAINTAINED FREE OF GAS AND THE CELLS OF THE CURED SPONGE ARE FILLED WITH VAPORS OF THE ORGANIC LIQUID BLOWING AGENT, WHICH ON COOLING TO BELOW IT BOILING POINT CONDENSES TO A LIQUID THAT DISSOLVES IN THE CURED SPONGE POLYMER.

United States Patent @fice Patented Oct. 10, 1972 3,697,455 METHOD FOR MAKING CELLULAR RUBBERY POLYMER Lawrence Spenadel, Elizabeth, Homer J. Hall, Cranford, and Isidor Kirshenbaum, Westfield, N.J., assiguors to Esso Research and Engineering Company No Drawing. Filed June 22, 1959, Ser. No. 822,713

Int. Cl. C08c 17/10 US. Cl. 260-25 R 6 Claims The present invention relates to a novel method for making cellular substances which are useful in rocket propellant systems. Specifically, it involves the use of compatible organic blowing agents in the preparation of sponges that have substantially uniform cells. In one aspect of the invention the sponge is blown in such a manner that there is little, or no, air in the cells of the finished sponge. In another aspect of the invention, a substantially gas-free organic blowing agent is employed which volatilizes at a temperature which is lower than, but close to, the temperature at which the sponge is cured.

It is known in the art of hybrid rocket propellants that sponges may be compounded with fuels and filled with liquid oxidizing agents and used to propel rocket devices. A rocket propulsion device as used herein is a rigid container for matter and energy so arranged that a portion of the matter can absorb the energy in kinetic form and subsequently eject it in a specified direction. The type of rocket to which the invention has particular application is that which is generally designated as a pure rocket, that is to say, a rocket in which the means producing the thrust does not make use of the surrounding atmosphere. The rocket is propelled by contacting the fuel or propellant in a combustion chamber with a suitable oxidizing agent which produces burning and therefore causes the release of energy at a high but controllable rate from the combustion chamber. The solid bi-propellants normally used in rockets consist of solid fuel and solid oxidizing agents. While solid oxidizing agents are highly useful, they are usually less effective than liquid oxidizing agents because the latter, in most instances, permit attainment of higher specific impulses.

It is an object of the present invention to provide a novel method for making sponges. Another object is to describe a method for preparing sponges that possess outstanding physical properties in relationship to sponges made by other methods. Still another object of the invention is to prepare sponges which are practically air-free that can be suitably employed in rocket motors in which the presence of air presents a problem.

In accordance with the present invention a blowing agent, which is compatible with the polymer used in the manufacture of the sponge, is added to the sponge mix and the resulting mixture is subjected to blowing conditions which usually involves heating the compounded polymer to an elevated temperature and maintaining said temperature until a suitable sponge is formed. It has been found that by selecting the appropriate blowing agent, it is possible not only to produce a sponge which has uniform cells, i.e. no large pockets, but also one which has improved physical properties, especially tensile strength. A strong, elastic propellant is quite desirable in pure rockets which employ solid chemical motors to propel them. The solid propellants in such rockets undergo especially severe stresses, e.g. pressures of 300-1500 p.s.i., while burning in the combustion chamber which may cause them to crack and this may result in uneven burning and sometimes detonation. In order to avoid the foregoing occurrences, the solid propellant should have suflicient elasticity to contract and expand under the pressures built up in the combustion chamber, yet be strong enough to withstand the actions of the gas forces in the chamber.

Since the sponges of the present invention possess unusual physical properties, they are readily adaptable to use in rocket motors.

In selecting the appropriate blowing agent for a given sponge mix, consideration should be given to the conditions under which the sponge will be formed. For example, most rubbery open cell sponges are simultaneously blown and cured at temperatures of 120-205 C. under approximately atmospheric pressure for about 5 minutes to an hour or more. The choice of blowing agent will depend to a large extent upon the maximum temperature to which the sponge mix Will be heated. This temperature, which is sometimes referred to as the cure temperature, must be above the temperature at which the blowing agent vaporizes under the pressure employed. For instance, if the pressure is atmospheric, the blowing agent should have a boiling point which is at least 5 C. below the curing temperature. On the other hand, it has been noted that the best results are obtained when the volatilization or boiling point of the blowing agent is not more than C. below the cure temperature and preferably not more than 40 C. below that temperature. In terms of boiling point, the liquid blowing agent should boil above 70 C., otherwise very cold mixing temperatures are needed, and large gas pockets are formed in the finished sponge. Thus, for the purpose of the present invention the blowing agent should contain 3 to 10 carbon atoms and have a boiling point above 70 C. but not more than 200 C., the upper limitation being governed by the maximum cure temperature. When conventional cure temperatures, i.e. 140160 C., are employed, it is preferred to select a blowing agent having a boiling point of -l50 C.

Another important feature of the invention is the plasticizing properties of the blowing agent. Smooth blowing and improved physical properties are obtained only when the liquid blowing agent is compatible with the polymer in the sponge mix, that is to say it will dissolve in the polymer or swell it. Of course, it is possible to employ conventional plasticizing agents, e.g. hydrocarbon oils, but these materials tend to weaken the sponge and produce cellular substances which have inferior tensile strengths. Thus in one embodiment of the invention, the blowing agent serves a dual purpose, namely that of a blower and a plasticizer. Generally speaking, most of the polymers used to manufacture rubbery sponges fall within two classes, those which are considered primarily hydrocarbon polymers and those which are non-hydrocarbon polymers, i.e. polymers containing more than about 5 wt. percent of elements other than hydrogen and carbon.

Insofar as hydrocarbon polymer sponges are concerned, it has been found that the most suitable blowing agents are aromatic, naphthenic and parafiinic hydrocarbons having boiling points above 70 but not greater than 200 C. While the aromatic and naphthenic hydrocarbons may contain as few as 6 carbon atoms, the paraffinic hydrocarbons should contain at least 7 carbon atoms. The preferred carbon number range for all of the aforementioned types of hydrocarbons is C -C Among the liquid hydrocarbons which may be used in accordance with the present invention are benzene, toluene, orthoand para-xylene, cyclohexane, cycloheptane, cyclooctane, heptane, isooctane, octane, isononane and isoheptane.

For the non-hydrocarbon polymers, that have a carbon backbone, it is advisable to use halogenated or oxygenated hydrocarbon blowing agents. These substances are more compatible and therefore have better plasticizing properties particularly when they are compounded with halogenated polymers, such as Viton A, which is a trade name of E. I. du Pont de Nemours for hexatluoropropylene and vinylidene fluoride copolymer. The halogenated hydrocarbon blowing agents may be either partially halogenated or perhalogenated. However, if it is perhalogenated, it is advisable to select a polar blowing agent because the polymer is generally polar in nature. Suitable halogenated hydrocarbons include tetrachlorohexafluorobutane, trichloroheptafluorobutane, dichlorooctafiuorobutane, 1,3-dichloropropane, 2,3-dichloro-2-methylbutane and 1,2-dichloro-2- methylpropane. The preferred halogenated hydrocarbon blowing agents have 3 to 4 carbon atoms and contain either chlorine or fluorine or both.

Organic oxygenated hydrocarbons, particularly ketones and aldehydes, may also be used as blowing agents for non-hydrocarbon polymers. Among the cyclic and acyclic ketones and aldehydes which may be used in accordance with the present invention are methylethyl ketone, methylisobutyl ketone, methyl-n-propyl ketone, diethyl ketone, di-n-propyl ketone, cyclohexanone, diisopropyl ketone, chloral, n-valeraldehyde and n-heptaldehyde. The preferred oxygenated compounds are saturated substances that contain 4 to 8 carbon atoms.

In carrying out the present invention, one or more solid polymers, either of the hydrocarbon or non-hydrocarbon type, is admixed with conventional amounts of curing agents, accelerators and stabilizers which are known in the art. Usually the liquid organic blowing agent is the last ingredient added to the sponge mix, however, this is not necessarily the case in every instance. Moreover, where the sponge is to be used in a rocket motor, the fuel is generally added to the mix immediately after the polymer has been placed on the mill or in the mixer. If the polymer can be processed at room temperature or a slightly elevated temperature, it is preferred to do so, however, it may be necessary in some cases to compound the polymer at temperatures which are substantially above that of the room, e.g. up to 150 C.

The polymer is preferably a rubbery polymer, such as butyl rubber, natural rubber, styrene-butadiene rubber, polyisoprene, polybutadiene, chlorinated butyl rubber, brominated butyl rubber, polychloroprene and hexafluoropropylenevinylidene fluoride copolymer. The rubbery polymers should have a molecular weight which is sufficiently high, i.e. 100,000 to 1,500,000 viscosity average molecular weight, to impart structural strength to the sponge. They should also possess either unsaturated bonds or reactive groups which permit cross-linking or curing. While the polymer should consist mainly of a rubber polymer, minor proportions of resins, such as polyethylene and polypropylene, may be used to provide additional strength.

Butyl rubber, a term well known in the rubber art, e.g. 4

Chapter 24 in Synthetic Rubber edited by G. Whitby, is a rubbery copolymer comprising a major proportion of a monoolefin having 4 to 7 carbon atoms and a minor proportion of a multiolefin having 4 to 8 carbon atoms. The most commonly employed monoolefin is isobutylene, although other monoolefins such as 3-methyl-butene-l and 4-methyl-pentene-1 may be used. Suitable multiolefins, which are generally conjugated diolefins, include isoprene, butadiene-1,3- dimethyl butadiene-1,3, piperylene, methyl pentadiene and the like. Most of the copolymers contains about 90 to 99.5 wt. percent isoolefin and 0.5 to 10 wt. percent diolefin, which in 'most instances is isoprene. The polymerization is generally carried out at a low temperature, e.g. between and -l65 C., in the presence of a Friedel-Crafts catalyst, such as aluminum chloride, dissolved in a lower alkyl halide, such as methyl chloride, ethyl chloride, etc. Their preparation is fully described in U.S. Pat. 2,356,128. Butyl rubbers have Wijs iodine numbers between about 1 and 50. Chlorinated and brominated butyl rubbers, containing about 1 to 4 wt. percent halogen, have similar iodine numbers.

Styrene-butadiene rubber, which generally contains about 20 wt. percent styrene and 80 wt. percent butadiene- 1,3, is made by an emulsion polymeriation technique and has a Wijs iodine number above about 300, e.g. 425. Na-

'4 tural rubber, polyisoprene and polybutadiene also have a similar unsaturation.

Since the sponges of the present invention have special application in the field of rocket propellants, the following recipe for sponges coming within the purview of the invention includes a fuel that is capable of propelling a rocket device when combusted:

While liquid fuels, such as naphthenic or parafiinic hydrocarbons may be used, solid fuels are preferred because they generally produce more energy upon combustion. Among the solid fuels which may be admixed with the polymers are metals selected from Groups I-IV of the Periodic Chart of Elements (Langes Handbook of Chemistry, 8th edition, pages 56-57). In most instances the hydrides of these metals are equally elfective fuels. The most outstanding fuels are metal-containing compounds in which the metal is selected from Groups II and III. Suitable fuels include lithium hydride, aluminum hydride, decaborane, magnesium hydride, titanium hydride, titanium, beryllium, magnesium, lithium and especially and especially aluminum and boron.

Accelerating agents, such as tellurium diethyldithiocarbamate and tetramethylthiuramdisulfide, may be employed to assist the curing agent which is usually sulfur or a sulfur-bearing compound in hydrocarbon rubber sponge mixes. Aside from sulfur, curing agents such as phosphorus compounds, para-dinitrosobenzene, paraquinone dioxime and polymethylol para-substituted phenolic resins may be employed. Any conventional curing agent can be used if it is capable of cross-linking the polymer molecules at the above-mentioned curing temperatures. It is sometimes desirable to employ a Group II metal oxide, such as zinc oxide or magnesium oxide, in order to obtain outstanding physical properties in the cured sponge. These oxides also act as curing agents when mixed with halogenated polymers.

One advantage of the present invention is that it provides a method for making air-free sponges which can be employed as supports for fuels and liquid oxidizing agents. The presence of even very small quantities of air entrapped in a closed cell, partially filled cell or pore in the hybrid propellant can cause uneven burning and hot spots. Moreover, the cells are of somewhat uniform size and this insures an adequate dispersion of the oxidizing agent in the fuel in the matrix of the sponge. Since the organic blowing agents of the present invention may contain a certain amount of dissolved gas, e.g. air, it is advisable to use a freshly distilled sample of the liquid, preferably one which has been condensed in a gas-free system.

The open cell sponges prepared in accordance with the invention are readily filled with liquid oxidizing agents by submerging them in the liquids and permitting them to absorb them. Steps should be taken to avoid contamination by air or other gases where the presence of such gases is a problem. For instance, the sponge may be prepared and maintained in an environment consisting of the blowing agent, e.g. atmosphere of the vaporized blowing agent, until it is filled with the liquid oxidizing agent and placed in the burning chamber of the rocket. The liquid oxidizing agent which is selected should be chosen not only on the basis of its reactivity with the particular fuel, but also on the basis of its reactivity with the sponge itself. That is to say, it should be inert insofar as the sponge is concerned and not attack it or destroy its structure prior to ignition. Among the liquid oxidizing agents which may be used in the hybrid propellants prepared with the sponges of the present invention are hydrogen peroxide, white fuming nitric acid, red fuming nitric acid, tetranitromethane, nitromethane-hydrazine, hydroxylamine, and bromotrifiuoride. The preferred oxidizing agents are inorganic and organic compounds containing either nitrogen or oxygen or both. The organic oxidizers generally have a low carbon number, that is to say about 1 to 4 carbon atoms.

In order for the sponge filled with fuel and oxidizing agent to be classified as a high energy propellant, it must contain about 75-95 wt. percent fuel and oxidizing agent. The matrix or intercellular substance (exclusive of the fuel) should not comprise more than about 25 wt. percent and preferably less than wt. percent of the' total propellant.

The following examples are submitted in order to illustrate some specific embodiments of the present invention.

EXAMPLE 1 Isobutylene-isoprene butyl rubber having a viscosity average molecular weight of about 400,000 and a mole percent unsaturation of about 1.5 is compounded according to the following recipe on a cold laboratory mill.

Recipe Component: Parts by weight Butyl rubber 100 Aluminum powder 100 Zinc oxide l2 Stearic acid 1 Sulfur 5 Tetramethylthiuramdisulfide 2.5 Benzothiazyldisulfide 1.5 n-Octane 10 The compounded butyl rubber is then calendered and a sheet 6 x 6 x is placed in an air-tight aluminum mold at 155 C. and is maintained at that temperature in the mold for 30 minutes. It is noted that the open cell sponge which is recovered has uniform size cells, a density of 0.4 gm./cc. and a tensile strength of 35 p.s.i. A similar sponge of the same density made with sodium bicarbonate as the blowing agent has a tensile strength of about 10-15 p.s.i.

A strand sponge 6 x A x A" is prepared in the same way, filled with 98% hydrogen peroxide by immersion at room temperature and burned under atmospheric pressure. It is noted that the burning rate is about 1 in./sec. and white sparks are produced.

EXAMPLE 2 The isobutylene-isoprene butyl rubber employed in Example 1 is compounded according to the following recipe:

Recipe Components: Parts by weight Butyl rubber 100 Boron powder 31 Zinc oxide l2 Stearic acid 1 Sulfur 5 Tetramethylthiuramdisulfide 2.5 Benzothiazyldisulfide 1.5 Isooctane The ingredients are mixed on a cold rubber mill and cured and blown under the conditions used in the previous example. It is noted that the open cell sponge which is formed has uniform size cells, a density of 0.48 gm./cc. and a tensile strength of 40 p.s.i.

A strand sponge is prepared in the same way, immersed in tetranitromethane at room temperature and the strand which contains 72 Wt. percent of the oxidizing agent burns at the rate of 1 in./sec. under atmospheric pressure with a green flame.

6 EXAMPLE 3 Example 1 is repeated except that the recipe contains 70 parts of butyl rubber and 30 parts of linear polypropylene having a molecular weight of 200,000 and a melting point of 265 C.

EXAMPLE 4 Viton A, which has a Mooney viscosity ML-4 min. at C. of 67 and a specific gravity of 1.82, is compounded according to the following recipe and blown with a perhalogenated alkane.

Recipe Components: Parts by weight Viton A 100 Boron powder 40 Hexamethylenediamine carbamate 1.5 Dibasic lead phosphite 10 Zinc oxide 10 Tetrachlorohexafluorobutane 15 The ingredients are compoundsd on a cold laboratory mill and cured in a closed mold at C. for 30 minutes. The air-free open cell sponge which is recovered has a density of 0.45 gm./cc., a tensile strength of 50 p.s.i., and when a strand sponge made in the same manner and containing 60 wt. percent of tetranitromethane is burned under atmospheric pressure, the burning rate is /z in./ sec. and the flame is colored green.

EXAMPLE 5 Example 4 is repeated with the exception that only 30 parts by weight of boron powder is employed and the blowing agent is di-n-propyl ketone. The open cell sponges prepared with the aforementioned blowing agent has uniform cells and a tensile strength of 50 p.s.i. A Viton A sponge of identical composition, which is blown with a conventional blowing agent, has a tensile strength of about 25 p.s.i.

EXAMPLE 6 Example 4 is repeated using n-valeraldehyde as the blowing agent.

EXAMPLE 7 The recipe in Example 4 is used to compound a Viton A sponge on a cold laboratory mill, cure and blow it at 155 C. for 30 minutes, with the exception that the compounded sponge mix does not contain any fuel (boron powder). A sponge having uniform size cells is obtained.

The densities of the sponges prepared in accordance with the present invention may vary from 0.25 to 0.5 gm./cc., however, in most instances the density will be between 0.28 and 0.48 gm./cc. The density of a given sponge is determined by the ingredients in the recipe as well as the amount of sponge mix placed in the mold. It is generally desirable to have between about 40-85% voids in the sponge and this may be achieved by filling the mold in which the sponge is blown and cured with no more than one-half the volume of the mold, i.e. there is room for at least 50% expansion.

Resort may be had to various modifications and variations of the present invention without departing from the spirit of the discovery or the scope of the appended claims.

What is claimed is:

1. Method for making a gas-free cellular rubbery polymer sponge having open cells which comprises, cold mixing and milling a curable rubbery polymer, a curing agent for curing said polymer, and 5 to 30% by weight of an organic liquid blowing agent based on the weight of the polymer, said organic liquid blowing agent being a liquid having a boiling point in the range of 70 to 200 C. and at least 5 C. below the curing temperature of the polymer, and being selected from the group consisting of aromatic hydrocarbons, naphthenic hydrocarbons,

parafiinic hydrocarbons, fluorine-substituted paraffins, chlorine-substituted parafiins, and ketones free of dissolved gas and compatible with the polymer to compound the rubbery polymer with the curing agent and said organic liquid blowing agent dissolved into the polymer as plasticizer, curing the resulting compounded polymer at a curing temperature in the range of 120 to 205 C. with simultaneous blowing of the polymer undergoing curing by volatilization of said organic liquid blowing agent under about atmospheric pressure for about five minutes to an hour, the curing and blowing of the compounded polymer being thus carried out in a mold cavity in which there is room for at least 50% expansion so that the volatilized organic liquid blowing agent forms open cells in the resulting sponge matrix of the cured polymer maintained free of gas and the cells of the cured sponge are filled with vapors of the organic liquid blowing agent, which on cooling to below its boiling point condenses to a liquid that dissolves in the cured sponge polymer.

2. The method according to claim 1, in which the rubbery polymer is a hydrocarbon polymer and the organic liquid blowing agent is a parafiinic hydrocarbon containing 6 to 10 carbon atoms per molecule.

3. The method according to claim 1, in which the rubbery polymer is isobutylene-isoprene butyl rubber and the organic liquid blowing agent is a paraflinic hydrocarbon having 8 carbon atoms per molecule.

4. The method according to claim 1, in which the References Cited UNITED STATES PATENTS 2,383,110 8/1945 Cooper 260722 2,313,154 3/1943 Kelly 260722 2,740,702 4/1956 Mace 520.5 2,857,258 10/1958 Thomas 52-0.5 2,861,898 9/1958 Platzer 2602.5 X 2,806,073 9/1957 Curtis et a1. 260722 X 2,763,208 9/1956 Rockoff et a1. 260722 X 2,977,329 3/1961 Lecher et a1. 2602.5 2,983,692 5/1961 DAlelio 2602.5

LELAND A. SEBASTIAN, Primary Examiner US. Cl. X.R. 

1. METHOD FOR MAKING A GAS-FREE CELLULAR RUBBERY POLYMER SPONGE HAVING OPEN CELLS WHICH COMPRISES, COLD MIXING AND MILLING A CURABLE RUBBERY POLYMER, A CURING AGENT FOR CURING SAID POLYMER, AND 5 TO 30% BY WEIGHT OF AN ORGANIC LIQUID BLOWING AGENT BASED ON THE WEIGHT OF THE POLYMER, SAID ORGANIC LIQUID BLOWING AGENT BEING A LIQUID HAVING A BOILING POINT IN THE RANGE OF 70* TO 200* C. AND AT LEAST 5*C. BELOW THE CURING TEMPERATURE OF THE POLYMER, AND BEING SELECTED FROM THE GROUP CONSISTING OF AROMATIC HYDROCARBONS. NAPHTHENIC HYDROCARBONS. PARAFFINIC HYDROCARBONS, FLUORINE-SUBSTITUTED PARAFFINS, CHLORINE-SUBSTITUTED PARAFFINS, AND KETONES FREE OF DISSOLVED GAS AND COMPATIBLE WITH THE POLYMER TO COMPOUND THE RUBBERY POLYMER WITH THE CURING AGENT AND SAID ORGANIC LIQUID BLOWING AGENT DISSOLVED INOT THE POLYMER AS PLASTICIZER, CURING THE RESULTING COMPOUNDED POLYMER AT A CURING TEMPERATURE IN THE RANGE OF 120* TO 205*C. WITH SIMULTANEOUS BLOWING OF THE POLYMER UNDERGOING CURING BY VOLATILIZATION OF SAID ORGANIC LIQUID BLOWING AGENT UNDER ABOUT ATMOSPHERIC PRESSURE FOR ABOUT FIVE MINUTES TO AN HOUR, THE CURING AND BLOWING OF THE COMPOUNDED POLYMER BEING THUS CARRIED OUT IN A MOLD CAVITY IN WHICH THERE IS ROOM FOR AT LEAST 50% EXPANSION SO THAT THE VOLATILIZED ORGANIC LIQUID BLOWING AGENT FORMS OPEN CELLS IN THE RESULTING SPONGE MATRIX OF THE CURED POLYMER MAINTAINED FREE OF GAS AND THE CELLS OF THE CURED SPONGE ARE FILLED WITH VAPORS OF THE ORGANIC LIQUID BLOWING AGENT, WHICH ON COOLING TO BELOW IT BOILING POINT CONDENSES TO A LIQUID THAT DISSOLVES IN THE CURED SPONGE POLYMER. 